The idling-stop vehicle is known which performs an idling-stop for automatically stopping an engine for the time being when the vehicle stops due to the wait at traffic lights or the like. JP 2002-38984 discloses an arrangement in such an idling-stop vehicle in which a booster circuit for boosting the battery voltage is provided between the battery and an electrical load. The booster is activated when the battery voltage falls below a first set value due to an operation of a starter at the time of an engine start condition being met. The booster is stopped when the battery voltage increases above a second set value due to an operation of an alternator after the completion of the engine start.
JP 2005-237149 discloses a technique directed to reducing such a change in the output voltage before and after the operation of the booster, wherein when the booster is activated in connection with the restart of the engine after the idling-stop, a target voltage of the booster is set based on the battery voltage immediately before the activation of the booster. Further, JP 2005-237149 discloses another embodiment in which the target voltage of the booster circuit is set in such a manner that voltage of the booster circuit is reduced gradually over a time with a predetermined voltage drop rate from the battery voltage immediately before the activation of the booster to a predetermined lower limit voltage.
At the time of a restart of an engine after an idling-stop, it is possible to improve stability of the operation of the load provided on the output side of the booster if the booster boosts the battery voltage which otherwise would be reduced greatly due to the operation of the starter. However, since the battery voltage is reduced during the idling-stop, there could be a large variation in an output voltage of the booster circuit at the beginning of the boosting operation if a manner of setting the target voltage of the booster circuit at the beginning of the boosting operation is not appropriate. Further, if the load is low at the time of the completion of the cranking of the engine, the battery voltage (and output voltage of the alternator) increases rapidly. Thus, there could be a large variation in an output voltage of the booster circuit at the time of such a rapid increase in the battery voltage, if a manner of setting the target voltage of the booster circuit at the time around the completion of the cranking is not appropriate. If such a large variation in an output voltage of the booster circuit occurs, problems such as blinking of a meter display installed in an instrument panel and lighting equipment such as a head light, etc., occur.